1. Field of the Invention
This invention relates to converting alternating current to direct current. In one of its aspects, the invention relates to a resistance welding apparatus utilizing Scott-connected transformers for converting three-phase alternating current to direct current.
2. State of the Prior Art
DC welding systems are typically provided by converting three-phase electrical energy to direct current through a three-phase transformer with rectified outputs. For example, see U.S. Pat. Nos. 2,221,576 to Dawson (issued Nov. 12, 1940), Aldenhoff 3,339,107 (issued Aug. 29, 1967), Riley 3,460,022 (issued Aug. 5, 1969) and Wolf 3,840,720 (issued Oct. 8, 1974). Such three-phase systems are bulky and require expensive controls to control the firing input or output voltages to produce a smooth output voltage pattern with substantially constant conduction and minimum ripple effect.
Sciaky, in U.S. Pat. No. 3,495,067 (issued Feb. 10, 1970), discloses the use of Scott-connected transformers to convert three-phase alternating current to direct current. Conductive coils are provided in the secondary circuit for adjusting the inductance of the conductors in the secondary circuit so as effect the correct commutation of current from one phase to the next in the primary. This system is expensive, cumbersome and, without a precise timed primary is somewhat inaccurate as a balancing means.
In welding apparatus, the secondary output terminals from the transformers are typically connected to diode assemblies which rectify the output current. The direct current terminals of the diodes are connected together so that the common output from the diodes has a relatively constant DC voltage during the welding cycle. In some cases, multiple diodes are connected to the secondary output terminals. Different conductor lengths may result in unequal current division among the rectifier cells in any given array. Riley, in U.S. Pat. No. 3,460,022 (issued Aug. 5, 1969), discloses the use of a looped AC connector in the form of a rectangular frame to enhance equal current division among diode cells. Further, slots are used in the frame to further enhance current division along any given row of such diode cells. Flexible braided leads are provided between the diodes and the rectangular frame. These braided leads have limited current carrying capacity. Therefore, multiple diodes must be used when these braided connectors are used.